Display device

ABSTRACT

The purpose of the present invention is to decrease the resistance of the drain and source in the TFT of the oxide semiconductor as well as to have stable Vd-Id characteristics of the TFT. The structure of the present invention is as follows: A display device having plural pixels including thin film transistors (TFT) having oxide semiconductor films comprising: a gate insulating film formed on the oxide semiconductor film, an aluminum oxide film formed on the gate insulating film, a gate electrode formed on the aluminum oxide film, a side spacer formed on both sides of the gate electrode, and an interlayer insulating film formed on the gate electrode, the side spacer, a drain and a source, wherein in a plan view, and in a direction from the drain to the source, a length of the gate electrode is shorter than a length of the aluminum oxide film.

The present application is a continuation application of InternationalApplication No. PCT/JP2018/043661, filed on Nov. 28, 2018, which claimspriority to Japanese Patent Application No. 2018-002641, filed on Jan.11, 2018. The contents of these applications are incorporated herein byreference in their entirety.

BACKGROUND OF THE INVENTION (1) Field of the Invention

The present invention relates to the display device using oxidesemiconductor TFTs.

(2) Description of the Related Art

The organic EL display device has the driving transistor and theswitching transistor, which are formed by TFT (Thin Film Transistor), inthe pixel; the liquid crystal display device has a switching transistorin the pixel. Therefore, the characteristics of the TFT is important.

The oxide semiconductor has high OFF resistance, therefore, OFF currentcan be made low when it is used in the TFT. Consequently, a fluctuationin the pixel voltage can be made low. In addition, the TFTs using theoxide semiconductor can be formed in lower temperature in themanufacturing process than that of the TFTs using the polysiliconsemiconductor; thus, the display device using the resin substrate can berealized.

In the TFT, the electric field is concentrated at region between thechannel and the drain; thus, there is a chance that a break down occursin this region. Therefore, in the TFT that uses the polysiliconsemiconductor, forms the LDD (Lightly Doped Drain) region between thechannel and the drain to avid the break down.

In the TFT of the oxide semiconductor, hydrogen is supplied to the drainregion and the source region to give conductivity in those regions.Patent document 1 discloses to diffuse the hydrogen, which is in thedrain and the source, into the region between the channel and the drainor the source to form the same effect as LDD, namely, to form the regionthat has lower resistance than that of the channel and higher resistancethan that of the drain or the source.

PRIOR ART DOCUMENTS

Patent document 1:

Japanese patent application laid open No. 2017-85079

SUMMARY OF THE INVENTION

Forming the medium resistance region by diffusing hydrogen under thegate electrode, as disclosed in patent document 1, is difficult tocontrol the diffusing area of the hydrogen. Especially, when the channellength is short, there is a danger that the TFT is depleted. If morehydrogen is supplied to the drain and the source to decrease theirresistance, the danger of depletion of the TFT increases.

The purpose of the present invention is to form stably the intermediateregion between the channel and the drain or the source. Another purposeof the present invention is to form the TFT of the oxide semiconductorhaving stable characteristics.

The present invention overcomes the above explained problem; theconcrete structures are as follows.

(1) A display device having plural pixels including thin filmtransistors (TFT) having oxide semiconductor films comprising:

a gate insulating film formed on the oxide semiconductor film,

an aluminum oxide film formed on the gate insulating film,

a gate electrode formed on the aluminum oxide film,

a side spacer formed on both sides of the gate electrode,

an interlayer insulating film formed on the gate electrode, the sidespacer, a drain and a source,

wherein in a plan view, and in a direction from the drain to the source,a length of the gate electrode is shorter than a length of the aluminumoxide film.

(2) The display device according to (1),

wherein, in a plan view, a channel is formed in the oxide semiconductorfilm at a place where the gate electrode contacts with the aluminumoxide film and overlaps with the oxide semiconductor film, theintermediate region is formed in the oxide semiconductor film at a placewhere the side spacer contacts with the aluminum oxide film and overlapswith the oxide semiconductor film.

(3) The display device according to (1),

wherein the interlayer insulating film directly contacts with the drainand the source of the oxide semiconductor film.

(4) The display device according to (1),

wherein the drain and the source of the oxide semiconductor film arecovered by the gate insulating film, the gate insulating film contactswith the interlayer insulating film.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of the organic EL display device;

FIG. 2 is a cross sectional view of the display area of the organic ELdisplay device as a comparative example;

FIG. 3 is a cross sectional view that shows a problem in the structureof FIG. 2;

FIG. 4 is a cross sectional view of the display area of the organic ELdisplay device according to the present invention;

FIG. 5 is a cross sectional view of the TFT portion according toembodiment 1;

FIG. 6 is a plan view of the TFT portion according to embodiment 1;

FIG. 7 is an interim cross sectional view in the manufacturing processof the TFT according to the present invention;

FIG. 8 is an interim cross sectional view that follows the process ofFIG. 7;

FIG. 9 is an interim cross sectional view that follows the process ofFIG. 8;

FIG. 10 is an interim cross sectional view that follows the process ofFIG. 9;

FIG. 11 is an interim cross sectional view that follows the process ofFIG. 10;

FIG. 12 is an interim cross sectional view that follows the process ofFIG. 11;

FIG. 13 is a cross sectional view of the organic EL display deviceaccording to embodiment 2;

FIG. 14 is an interim cross sectional view in the manufacturing processof the TFT according to embodiment 2;

FIG. 15 is a cross sectional view of the completed TFT according toembodiment 2;

FIG. 16 is a plan view of the liquid crystal display device;

FIG. 17 is a cross sectional view of the liquid crystal display deviceaccording to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will be explained in detail referring to thefollowing embodiments. Examples of the oxide semiconductors are indiumgallium zinc oxide (IGZO), indium tin zinc oxide (ITZO), zinc oxidenitride (ZnON), and indium gallium oxide (IGO). Among the oxidesemiconductors, optically transparent and amorphous materials are calledTAOS (Transparent Amorphous Oxide Semiconductor). The oxidesemiconductor may be called TAOS in this specification. In embodiments 1and 2, the present invention is explained when it is applied in theorganic EL display device; in embodiment 3, the present invention isexplained when it is applied in the liquid crystal display device.

Embodiment 1

In FIG. 1, the scan line driving circuits 80 are formed at both sides ofthe display area 10. In the display area 10, the scan lines 91 extend inthe lateral direction (x direction) and are arranged in the longitudinaldirection (y direction). The video signal lines 92 and the power lines93 extend in the longitudinal direction and are arranged in the lateraldirection. The area surrounded by the scan lines 91 and the video signallines 92 or the power lines 93 is the pixel 95; the driving transistorand the switching transistor, which are made of TFTs, the organic ELlayer, which emits light, and so forth are formed in the pixel 95.

The terminal area 20 is formed at the one side of the TFT substrate 100.The flexible wiring substrate 600 is connected to the terminal area 20to supply powers and signals to the organic EL display device. If theTFT substrate 100 is made of glass of 0.2 mm or thinner, the displaydevice can be curved. Further, if the TFT substrate 100 is made of resinas polyimide and so forth, the flexible display device can be formed.Polyimide has superior characteristics for the substrate of the displaydevice because of its mechanical strength and heat resistance and soforth.

FIG. 2 is a cross sectional view of the display area 10 of the organicEL display device as a comparative example. The undercoat film 101 isformed on the TFT substrate 100. The role of the undercoat film 101 isto prevent the oxide semiconductor film 105, which is formed above theundercoat film 101, from being contaminated by impurities in the glasssubstrate or the resin substrate; another role is to strengthen theadhesiveness between the films in the display device and the resinsubstrate or the glass substrate.

The undercoat film 101 is e.g. a laminated film of three layers ofsilicon oxide film (herein after represented by SiO), silicon nitridefilm (herein after represented by SiN), and SiO film. The lower most SiOfilm prevents the intrusion of the impurities and maintains adhesivenesswith the resin substrate 100 or the polyimide substrate 100 as the TFTsubstrate 100. The SiN film has a superior barrier characteristicagainst the impurities, specifically moisture, from the glass substrate100 or the resin substrate 100. The upper most layer of SiO film has arole as a barrier against the impurities and improves adherence betweenthe layers formed on the SiO film and the substrate 100.

In FIG. 2, the bottom gate electrode 102 is formed on the undercoat film101. The gate electrode 109 is also formed over the oxide semiconductorfilm 105, consequently, the TFT in FIG. 2 is a so called dual gate TFT.In the structure of FIG. 2, the influence of the top gate 109 isstronger than that of the bottom gate 102. The bottom gate 102 also hasa role as a light shading effect against the light from the rear side.

The bottom gate insulating film, which has a two layer structure, isformed between the bottom gate electrode 102 and the oxide semiconductorfilm 105; the bottom gate insulating film of two layer structure has thefirst bottom gate insulating film 103 and the second bottom gateinsulating film 104. The first bottom gate insulating film 103 is madeof e.g. the silicon nitride (SiN) film of a thickness of 50 nm; thesecond bottom gate insulating film 104 is made of e.g. the silicon oxide(SiO) film of a thickness of 200 nm.

The oxide semiconductor film 105 is formed on the second gate insulatingfilm 104. The top gate insulating film 106 is formed on the channel ofthe oxide semiconductor film 105; the top gate electrode 109 is formedon the top gate insulating film 106. The structure of the TFT in FIG. 2is explained in FIG. 3.

In FIG. 2, the interlayer insulating film 110 is formed covering theTFT. The interlayer insulating film 110 is made of the SiN film or theSiO film, or a laminated film of the SiN film and the SiO film. Theinterlayer insulating film 110 in FIG. 2 has a role to supply hydrogento the oxide semiconductor film 105, therefore, it is preferable thatthe interlayer insulating film 110 contains hydrogen.

The through holes are formed to connect the drain region of the oxidesemiconductor 105 and the drain electrode 111, and to connect the sourceregion of the oxide semiconductor 105 and the source electrode 112. Theorganic flattening film 113 is formed by e.g. acrylic resin covering theinterlayer insulating film 110, the drain electrode 111, sourceelectrode 112 and so forth. Since the organic flattening film 113 has arole as a flattening film, it is formed thick as 1.5 to 4 microns.

The through hole is formed in the organic flattening film 113 to connectthe lower electrode 114 and the source electrode 112. The lowerelectrode 114 is a laminated film of the lower layer of the reflectionfilm made of e.g. silver and the upper layer of the anode for theorganic EL layer. The anode is made of e.g. ITO (Indium Tin Oxide) film,which is a transparent conductive film.

The bank 115 is formed covering the edge of the lower electrode 114 andthe organic flattening film 113, etc. The bank 115 is made of resin asacrylic. The role of the bank 115 is to form a step coverage to preventthe breaking of the organic EL layer 116 at the edge of the lowerelectrode 114 as well as to partition the pixels 95. The bank 115 isformed as that the material for the bank 115 is formed on all over thedisplay area 10, and subsequently, holes are formed at the areas wherethe organic EL layers are formed, namely, the light emitting areas areformed.

In FIG. 2, the organic EL layer 116 is formed on the lower electrode 114in the hole of the bank 113. The organic EL layer 116 comprises e.g.,from lower side, the hole injection layer, the hole transport layer, thelight emitting layer, the electrode transport layer, and the electroninjection layer.

The upper electrode 117, which is a cathode, is formed on the organic ELlayer 116 by a transparent electrode. The upper electrode 117 needs tobe transparent. The upper electrode 117 is made of the transparentconductive film of e.g. ITO, IZO (Indium Zinc Oxide), AZO (Antimony ZincOxide) and so forth or a thin film of metal as silver, etc. When themetal is made to thin film, it becomes transparent. The upper electrode117 is formed all over the display area 10 in common to the pixels.

The organic EL layer 116 is weak to moisture, and is mechanically weakbecause it is thin. Therefore, the protective film 118, which is alaminated film of the SiN film, the SiO film and the organic film madeof as e.g. acrylic resin, is formed covering the upper electrode 117.The SiN film works as a barrier against moisture, the organic film worksas a mechanical buffer, and the SiO film works as a barrier as well asfor maintaining adhesive strength with other layers.

Since the organic EL display device has reflection electrodes, theexternal light is reflected. Therefore, the organic EL display devicehas the circular polarizing plate 120 adhered to the display surface viathe adhesive 119 to prevent the reflection of the external light.

FIG. 3 is an enlarged view of the TFT portion in FIG. 2. In FIG. 3, thebottom gate electrode 102 made of metal is formed on the undercoat film101. The first bottom gate insulating film 103 is formed by SiN in athickness of e.g. 50 nm covering the bottom gate electrode 102. Thesecond bottom gate insulating film 104 is formed by SiO in a thicknessof e.g. 200 nm on the first bottom gate insulating film 103. The firstbottom gate insulating film 103 and the second bottom gate insulatingfilm 104 can be formed continuously by CVD (Chemical Vapor Deposition).

The oxide semiconductor film 105 is formed on the second bottom gateinsulating film 104. The characteristics of the semiconductor film 105change when it is reduced by hydrogen. Since the SiN film dischargeshydrogen, the second bottom gate insulating film 104, which contacts theoxide semiconductor film 105, is formed by SiO. The oxide semiconductorfilm 105 is formed by e.g. sputtering in a thickness of 10 to 100 nm.The oxide semiconductor film 105 is made of e.g. IGZO.

In FIG. 3, the top gate insulating film 106 is formed by SiO on thechannel region of the oxide semiconductor film 105 in a thickness ofe.g. 100 nm. The top gate insulating film 106 is formed as that: the SiOfilm is formed all over the display area and is left only at the placecorresponding to the channel region of the oxide semiconductor film 105.

After that, the top gate electrode 109 is formed on the top gateinsulating film 106. The thickness of the top gate electrode 109 is e.g.200 nm. The thickness of the top gate insulating film 106 is 100 nm,while the bottom gate insulating film is a laminated film of the secondbottom gate insulating film 104 made of SiO of a thickness of 200 nm andthe first bottom gate insulating film 103 made of SiN of a thickness of50 nm; therefore, Vd-Id characteristics of the TFT are mainly controlledby the top gate electrode 109. Herein after, the top gate electrode 109may be simply called the gate electrode and the top gate insulating film106 may be simply called as the gate insulating film.

After that, the interlayer insulating film 110 is formed covering thegate electrode 109 and the oxide semiconductor film 105. In many cases,the interlayer insulating film 110 is a laminated film of the SiN filmas the lower layer and the SiO film as the upper layer. The reason whythe SiN film is used as the lower layer is to supply hydrogen to theoxide semiconductor film 105 to form the drain region and the sourceregion in the oxide semiconductor film 105. Namely, in the annealingprocess, hydrogen from the SiN film diffuses into the area of the oxidesemiconductor film 105 that is not covered by the gate electrode 109 togive conductivity to the oxide semiconductor film 105. Thus, the drainregion and the source region are formed in the oxide semiconductor film105. Subsequently, through holes are formed in the interlayer insulatingfilm 110 to connect the drain region of the oxide semiconductor film 105and the drain electrode 111, and to connect the source region of theoxide semiconductor film 105 and the source electrode 112.

On the other hand, the portion of the oxide semiconductor film 105 thatis covered by the gate electrode 109 keeps high resistance sincehydrogen does not diffuse into this portion. However, hydrogen from theSiN film, which is absorbed in the drain and source of the oxidesemiconductor film 105 diffuses in lateral direction in the oxidesemiconductor film 105 during the anneal process. Consequently, thereoccurs a problem that the channel becomes conductive when the channellength is short; even if the channel does not become conductive, theVd-Id characteristics fluctuate.

The present invention overcomes this problem. FIG. 4 is a crosssectional view of the display area 10 of the organic EL display deviceaccording to the present invention. The structure of TFT of the oxidesemiconductor 105 and the structure of the interlayer insulating film110 in FIG. 4 are different from those in FIG. 2. FIG. 4 is the same asFIG. 2 up to the oxide semiconductor film 105, which is formed on thesecond bottom gate insulting film 104, is patterned.

In FIG. 4, the gate insulating film 106 made of SiO is formed in thearea between the drain region and the source region of the oxidesemiconductor film 105; the aluminum oxide (herein after AlO) film 107is formed on the gate insulating film 106. It may be said that the SiOfilm 106 and the AlO film 107 constitute the gate insulating film,however, in this specification, for convenience, the SiO film is calledthe gate insulating film 106 and the aluminum oxide film is called AlOfilm 107.

The gate electrode 109 is formed by metal or alloy on the AlO film 107;the present invention characterizes in that the side spacers 108, madeof insulating material as e.g. SiN, are formed at the both sides of theAlO film 107. As explained later, the side spacers 108 securely separatethe channel from the drain or the source.

The gate electrode 109 and the gate insulating film 106 are patterned,the drain and the source of the oxide semiconductor film 105 are notcovered by the gate insulating film 106 and the gate electrode 109,consequently, they contact directly with the interlayer insulating film110. The interlayer insulating film 110 is formed by the SiN film or alaminated film of the SiN film and the SiO film. The feature of thepresent invention is that the SiN film contacts with the drain or thesource of the oxide semiconductor film 105. Thus, hydrogen is suppliedto the drain and the source of the oxide semiconductor film 105;consequently, the resistance of the drain and the source can bedecreased.

Since hydrogen is easy to diffuse, when hydrogen is supplied to thedrain and the source, hydrogen diffuses into the channel during theannealing process, in the structure of FIG. 3, which is a comparativeexample; consequently, characteristics of the TFT become unstable.Further, if the channel is short, there is a danger that the channelbecomes conductive. The present invention prevents the TFT of the oxidesemiconductor from becoming unstable by forming side spacers 108 at bothsides of the gate electrode 109. The structures above the interlayerinsulating film are the same as explained in FIG. 2.

FIG. 5 is an enlarged cross sectional view of the TFT in FIG. 4. Thegate insulating film 106, the AlO film 107 and the gate electrode 109are formed on the oxide semiconductor film 105. The side spacers 108 areformed between the edge of the gate electrode 109 and the edge of theAlO film 107. The channel region 1051 of the oxide semiconductor film105 corresponds to the region that the gate electrode 109 opposes to theoxide semiconductor film 105 without the side spacers 108. In otherwords, the channel region 1051 of the oxide semiconductor film 105corresponds to the recessed portion 1091 of the gate electrode 109.

The gate insulating film 106 is formed under the gate electrode 109only, and is not formed on the drain region and the source region of theoxide semiconductor film 105. After that, the interlayer insulating film110 is formed by the SiN film covering the gate electrode 109 and theoxide semiconductor film 105 and etc. Since hydrogen is supplied to theregion where the oxide semiconductor film 105 contacts with the SiNfilm, the resistance of the drain and the source of the oxidesemiconductor film 105 becomes low, thus, more drain current can flow.

Conventionally, the SiO film that has a lot of defects was brought intocontact with the oxide semiconductor film 105 to form the drain and thesource; however, supply of hydrogen from the SiO film is less comparedwith that from the SiN film, thus, the resistance of the drain and thesource could not made low enough; consequently, flowing of a large draincurrent was impossible. On the other hand, if the interlayer insulatingfilm 110 is formed by the SiN film in the conventional structure,hydrogen may be supplied to the oxide semiconductor film 105; however,the hydrogen diffuses into the channel 1051 of the oxide semiconductorfilm 105, too, and the channel 1051 is made conductive. This problem ismore dangerous in the TFT that the channel length is short.

The present invention controls the diffusion of hydrogen into thechannel 1051 by forming the side spacers 108 at the both sides of thegate electrode 109; and thus, realizes low resistance in the drain andthe source of the oxide semiconductor film 105 and, at the same time,suppresses the influence of hydrogen to the channel 1051. The presentinvention intensifies the effect by forming the AlO film 107 between thegate insulating film 106 and the gate electrode 109 and between the gateinsulating film 106 and the side spacers 108.

In FIG. 5, a thickness of the AlO film 107, which exists between thegate insulating film 106 and the gate electrode 109 and between the gateinsulating film 106 and the side spacers 108, is 10 to 50 nm. Supplyingoxygen is effective to suppress fluctuation of characteristics of thechannel in the oxide semiconductor film 105. The AlO film 107 also cansupply oxygen, therefore, the oxide semiconductor film 105 can besupplied with oxygen more stably.

The cross sectional view of the side spacer 108 formed on the AlO film107 is trapezoidal that one side wall has a tapered surface. The offsetregion 1052 is formed in the oxide semiconductor film 105. The offsetregion 1052 securely forms the intermediate region between the channeland the drain and between the channel and the source (herein after thesource is represented by the drain) in the oxide semiconductor film 105;even too much hydrogen is supplied to the drain or the source, theintermediate region prevents the channel from being influenced byhydrogen, thus, the fluctuation in characteristics of the TFT can beavoided. This is specifically effective when the channel length isshort.

In FIG. 5, a length d2 of the offset region 1052 is 0.5 to 2 microns.When the channel length d1 is e.g. 2 microns, a length d2 of the offsetregion 1052 is preferably approximately 1 micron. The AlO film 107exists between the offset region 1052 and the side spacer 108. Since theAlO film 107 is a good supplier of oxygen, the offset region 1052effectively offsets hydrogen that tries to diffuse into the channel fromthe drain, and thus, effectively prevents the channel from being reducedby hydrogen.

In FIG. 5, the edge of the gate electrode 109 extends on the top of theside spacer 108 to form the gate electrode 109 stably. The length d3 ofthe gate electrode 109 on the top of the side spacer 108 is preferably0.1 microns or more. The length d4 between the edge of the gateelectrode 109 and the outer edge of the offset region 1052 is preferably0.3 microns or more. The width of the offset region d2 is d3+d4.

In FIG. 5, one side surface of the side spacer 108 is a tapered surface.The reason why the tapered surface is made is to improve a step coveragefor the interlayer insulating film 110, which is formed covering thegate electrode 109 and the side spacers 108. Therefore, the taperedsurface of the side spacer 108 is not necessarily indispensable in thepresent invention.

A thickness of the side spacer 108 is e.g. 100 to 500 nm. If the sidespacer 108 is too thin, the side spacer 108 disappears during patterningby dry etching. On the other hand, if the side spacer 108 is too thick,the following problems occur: namely, a problem of a step coverage bythe interlayer insulating film 110, which covers the gate electrode 109and the side spacer 108, and a problem that manufacturing time of theside spacer 108 becomes too long, and so on. In addition, if the sidespacer 108 is too thick, when the side spacer is made of SiN, thereoccurs a problem that supplying hydrogen form the side spacer 108 itselfincreases.

The interlayer insulating film 110 is formed covering the gate electrode109 and the side spacer 108. The interlayer insulating film 110generally has a two-layer structure of the SiN film and the SiO film;the lower layer, which contacts with the oxide semiconductor film 105,is preferably the SiN film. The SiN film can be a supplier of hydrogen,thus, the resistance of the drain region and the source region of theoxide semiconductor 105 can be lowered.

In FIG. 5, the material for the side spacer 108 is preferably SiNbecause of requirement in microfabrication by dry etching as explainedlater; however, according to condition of fabrication, SiON (siliconnitride oxide) or SiO can be used. If the side spacer 108 is formed bySiN, the SiN in the interlayer insulating film and the SiN of the sidespacer 108 can be the same material. However, the amount of hydrogen inthe SiN of the interlayer insulating film 110 is larger than the amountof hydrogen in the SiN that constitutes the side spacer 108. Since theside spacer 108 is near to the channel, supplying hydrogen from the sidespacer 108 is better to be suppressed.

Both of the SiN films are formed by CVD; even if the same gas pressure,power, deposition pressure and so forth are the same in the CVD for thetwo SiN films, when the SiN film for the side spacer 108 is formed, thedeposition temperature is set higher, as e.g. 300 to 350 centigrade,than that for the SiN film for the interlayer insulating film 110. Sincehigh temperature deposition has higher clearance coating properties, ithas also merit that pores tend not to be included in the film.

FIG. 6 is a plan view of the TFT. In FIG. 6, the oxide semiconductorfilm 105 is formed on the bottom gate electrode 102 via the bottom gateinsulating film. The island shaped gate insulating film is formed on theoxide semiconductor film 105, however, it is not seen because it iscovered by the gate electrode 109 and the side spacer 108. In FIG. 6,the side spacer 108 is visible around the gate electrode 109; thiscorresponds to the tapered surface of the side spacer 108 in FIG. 5.

The recess 1091 is formed in the gate electrode 109. The channel 1051 ofthe oxide semiconductor film 105 is formed at the place corresponding tothe recess 1091 of the gate electrode 109; the offset region 1052 of theoxide semiconductor film 105 is formed at the both sides of the recess1091. In FIG. 6, the SiN film, which constitutes the interlayerinsulating film 110, contacts to the oxide semiconductor film 105 at theplace where the gate electrode 109 and the side spacer 108 do notoverlap; consequently, the oxide semiconductor film 105 is reduced byhydrogen supplied from the SiN film, thus, the drain and the source areformed. The drain electrode 111 connects with the drain via the throughhole and the source electrode 112 connects with the source via thethrough hole.

FIGS. 7 to 12 are interim cross sectional view in the manufacturingprocess for the structure of FIG. 5. FIG. 7 shows that: the bottom gateelectrode 102 is formed on the undercoat film 101; the first bottom gateinsulating film 103 and the second bottom gate insulating film 104 areformed covering the bottom gate electrode 102; and the oxidesemiconductor film 105 is formed on the second bottom gate insulatingfilm 104. The bottom gate electrode 102 is formed by sputtering, thenpatterned. The first bottom gate insulating film 103 is formed by SiN ina thickness of e.g. 50 nm; the second bottom gate insulating film 104 isformed by SiO in a thickness of e.g. 200 nm. The SiN film and the SiOfilm are continuously formed by CVD.

After that, the oxide semiconductor film 105 is formed on the secondbottom gate insulating film 104. The oxide semiconductor film 105 isformed in a thickness of 10 to 100 nm, then it is patterned in islandshape. Subsequently, top gate insulating film (gate insulating film) 106is formed by SiO on the oxide semiconductor film 105. A thickness of theSiO film is e.g. 100 nm. The relation between the bottom gate insulatingfilm and the top gate insulating film is explained in FIG. 2. Then, theAlO film 107 is formed by sputtering in a thickness of 10 to 50 nm.

FIG. 8 is an interim cross sectional view in the manufacturing processfor the side spacer 108. In FIG. 8, the SiN film 1081 is formed by CVDin a thickness of 100 to 500 nm for the side spacer 108. The CVD forthis SiN film is made at higher temperature (e.g. 300 to 350 centigrade)than the temperature of CVD for the SiN film for the interlayerinsulating film 110, which is formed later. The reason is to form denserfilm to suppress the release of hydrogen from the side spacer 108. InFIG. 8, the resist 400 is formed for patterning the side spacer 108.After that, SiN film is dry etched using SF6 base gas.

FIG. 9 is a cross sectional view that the SiN film for the side spacer108 is dry etched. The SiN film is easily side etched in dry etchingusing SF6 base gas. The side etching is shown by arrows in FIG. 9. Thewidth of the side spacer becomes narrower than the width of the resistdue to the side etching. Since the width of the side spacer 108 is verynarrow as 0.5 to 2 microns at the bottom, it is difficult to form aresist in such a narrow pattern. The width of the side spacer 108 can bemade less than the lower limit of patterning of the resist 400 by theeffect of side etching if the SiN film is used for the side spacer 108.

In comparing the AlO film 107 and the SiN film 108, which is toconstitute the side spacer 108, in dry etching of SF6 base gas, theselecting ratio of the dry etching between the AlO film 107 and the SiNfilm 1081 is so high; therefore, the AlO film 107 is not etchedessentially in a dry etching in FIG. 9.

After that as shown in FIG. 10, the metal or alloy for the gateelectrode 109 is formed and the resist 400 is formed at the place of thegate electrode 109. The materials for the gate electrode 109 are e.g.Mo, MoW, or a lamination film of e.g. Ti—Al—Ti. Subsequently, the AlOfilm 107 and the gate electrode 109 are patterned by dry etching usingChlorine base gas. When Chlorine base gas is used, the SiO film, whichconstitutes the gate insulating film 106 is not easily etched, however,not zero, therefore, the dry etching using Chlorine gas is stoppedduring the SiO film is being dry etched.

FIG. 11 is a cross sectional view that the gate electrode 109 and theAlO film 107 have been patterned by dry etching using Chlorine gas asshown in FIG. 10. In this state, the oxide semiconductor film 105 iscovered by the gate insulating film 106. Therefore, as shown in FIG. 11,the SiO film is patterned by dry etching using SF6 base gas to form thegate insulating film 106. The oxide semiconductor film 105 is notessentially etched by dry etching using SF6 base gas.

FIG. 12 is a cross sectional view that the gate insulating film 106 isformed by eliminating unnecessary SiO film by dry etching using SF6 basegas. The SiN that constitutes the side spacer 108 is also slightlyetched by dry etching using SF6 base gas, thus, the outer surface of theside spacer 108 is to have a taper.

In FIG. 12, the gate electrode 109 extends on the top side of the sidespacer 108, whose cross sectional view is trapezoidal. This amount(length) of the gate electrode 109 on the top of the side spacer 108 isd3, which is preferably 0.1 microns or more. The difference between thetop side and the bottom side of the side spacer 108 is preferably 0.3microns or more.

Subsequently, when the interlayer insulating film 110 is formed on thestructure of FIG. 12, the structure of FIG. 5 is realized. Theinterlayer insulating film 110 is formed by the SiN film or thelaminated film of the SiO film and the SiN film; when the laminated filmis used, the SiN film is preferably the lower layer, which contacts theoxide semiconductor film 105, to supply hydrogen to the oxidesemiconductor 105. The SiN that constitutes the interlayer insulatingfilm 110 preferably contains more hydrogen than that of the SiN thatconstitutes the side spacer 108.

Embodiment 2

FIG. 13 is a cross sectional view of the display area of the organic ELdisplay device according to embodiment 2. In FIG. 13, the top gateinsulating film 106 is formed not only under the gate electrode 109, butformed also all over the display area. In addition, the side surface ofthe side spacer 108 is not tapered. Other structures of FIG. 13 is thesame as FIG. 4. In FIG. 13, the interlayer insulating film 110 does notcontact with the drain region and the source region of the oxidesemiconductor film 105. Therefore, they cannot receive hydrogen from theinterlayer insulating film 110 even it is formed by SiN. Therefore, theion implantation (I/I) is performed to form the drain region and thesource region in the oxide semiconductor film 105 as shown in FIG. 14.

The cross sectional structure of FIG. 14 is the same as FIG. 11 ofembodiment 1. In FIG. 14, however, the dry etching is not performed, butthe ion implantation (I/I) is performed to give the conductivity to theoxide semiconductor film 105 by doping Boron (B) or Phosphorous (P) andso forth to form the drain and the source in the oxide semiconductorfilm 105. By the way, the ion implantation (I/I) gives the conductivityby forming defects in the structure of the oxide semiconductor film 105,thus, the atoms for the ion implantation can be Ar and so forth. The ionimplantation (I/I) is performed using the gate electrode 109 as themask. In addition, the side spacer 108 is made thick, and the AlO film107 exists in this area; therefore, the intermedium region is formedbetween the channel and the drain or the source. In the meantime, theside spacer 108 is not etched by the ion implantation (I/I), therefore,a tapered side surface, as depicted in FIG. 12, is likely not formed.

FIG. 15 is a cross sectional view that: after the ion implantation(I/I), the interlayer insulating film 110 is formed, through holes areformed in the interlayer insulating film 110, and drain electrode 111and the source electrode 112 are connected to the oxide semiconductorfilm 105 via the through holes. The structure of the interlayerinsulating film 110 is the same as explained in FIG. 5 and so forth inembodiment 1. In embodiment 2, the drain region and the source regionget conductivity through ion implantation (I/I).

The drain region and the source region of the oxide semiconductor film105 are covered by the gate insulating film 106, which has a lot ofoxygen. Therefore, it is possible that the resistance of the drainregion and the source region gradually increases even they are givenconductivity by ion implantation (I/I).

This phenomenon is mitigated by setting the SiN film of the interlayerinsulating film 110 in contact with the gate insulating film 106;namely, hydrogen from the SiN film of the interlayer insulating film 110mitigates the effect of oxygen from the gate insulating film 106 to thedrain and the source of the oxide semiconductor film 105. Therefore,characteristics of the TFT can be stabilized. Other structures are thesame as explained in embodiment 1.

Embodiment 3

In embodiment 1 and embodiment 2, the present invention is explainedwhen it is applied to the organic EL display device. The presentinvention, however, can be applied to the liquid crystal display device.FIG. 16 is a plan view of the liquid crystal display device. In FIG. 16,the TFT substrate 100 and the counter substrate 200 are adhered by theseal material 30; the liquid crystal is sealed inside of the sealmaterial 30.

The display area 10 is formed where the TFT substrate 100 and thecounter substrate 200 overlap. In the display area 10, the scan lines 91extend in the lateral direction (x direction) and are arranged in thelongitudinal direction (y direction); the video signal lines 92 extendin the longitudinal direction and are arranged in the lateral direction.The pixel 95 is formed in the area surrounded by the scan lines 91 andthe video signal lines 92. The TFT substrate 100 is made bigger than thecounter substrate 200; the area that the TFT substrate 100 does notoverlap with the counter substrate 200 is the terminal area 20. Theflexible wiring substrate 600 is connected to the terminal area 20 tosupply powers and signals to the liquid crystal display device.

FIG. 17 is a cross sectional view of the display area 10 of the liquidcrystal display device. In FIG. 17, the structure is the same up toforming the organic flattening film 110 as FIG. 4 in embodiment 1. Bythe way, the TFT in FIG. 4 in embodiment 1 is a driving TFT that drivesthe organic EL layer, and the TFT in this embodiment is a switching TFT,however, the basic structure is the same.

In FIG. 17, the two-layer structured bottom gate insulating film 103,104 is formed on the first gate electrode 102, which can work also asthe light shading film; the oxide semiconductor film 105 is formed onthe bottom gate insulating film 103, 104. The top gate insulating film106 is formed on the oxide semiconductor film 105 at the placecorresponding to the channel. The AlO film 107 is formed on the top gateinsulating film 106. Subsequently, the side spacers 108, whichcharacterize the present invention, are formed on the AlO film 107 atits both sides. After that the gate electrode 109 is formed.

The interlayer insulating film 110 is formed covering the gate electrode109, the side spacers 108, and the oxide semiconductor film 105 and soforth. The structure of the interlayer insulating film 110 is also thesame as in explained in FIG. 5. Subsequently, the through holes areformed in the interlayer insulating film 110 to connect the drain of theoxide semiconductor film 105 and the drain electrode 111 and to connectthe source of the oxide semiconductor film 105 and the source electrode112. The organic flattening film 113 is formed covering the drainelectrode 111, the source electrode 112 and the interlayer insulatingfilm 110.

In FIG. 17, the structures formed later than the organic flattening film113 are different from the organic EL display device of FIG. 4. FIG. 17is a cross sectional view of the liquid crystal display device of IPS(In Plane Switching) mode. In FIG. 17, the through hole is formed in theorganic flattening film 113 to connect the source electrode 112 and thepixel electrode 152. In FIG. 17, the common electrode 150 of the ITO isformed on the organic flattening film 113. The capacitance insulatingfilm 151 is formed by SiN on the common electrode 150.

The pixel electrode 152 is formed on the capacitance insulating film151. The pixel electrode 152 connects with the source electrode 112 viathe through hole formed in the organic flattening film 113. By the way,the capacitance insulating film 151 covers the side wall of the throughhole in the organic flattening film 113, however, the through hole isformed in the capacitance insulating film 151 at the bottom so that thepixel electrode 152 can connect with the source electrode 112. Thealignment film 153 is formed on the pixel electrode 152 to give aninitial alignment to the liquid crystal molecules 301.

The counter substrate 200 is formed opposing to the TFT substrate 100sandwiching the liquid crystal layer 300. On the inside surface of thecounter substrate 200, the black matrix 202 is formed, and the colorfilter 201 is formed at the place corresponding to the pixel electrode152. The overcoat film 203 is formed covering the black matrix 202 andthe color filter 201. The alignment film 204 is formed on the overcoatfilm 203 to give an initial alignment to the liquid crystal molecules301.

In FIG. 17, when the voltage is applied to the pixel electrode 152, thelines of force as shown by the arrows are generated to rotate the liquidcrystal molecules 301, consequently, light transmittance in the pixel iscontrolled and thus, the images are formed.

As described above, the TFT of oxide semiconductor having stablecharacteristics can be realized by applying the structure explained inembodiment 1. The structure of embodiment 2 can also be applied to theliquid crystal display device.

The above explanation is made for the IPS mode liquid crystal displaydevice; however, the present invention can be applied to other modes ofthe liquid crystal display device.

The TFT of the oxide semiconductor can make the leak current low,however, the mobility of the carriers is low compared with that of thepolysilicon semiconductor; therefore, sometimes, it may be difficult toform the peripheral driving circuit, as e.g. the scan line drivingcircuit 80, by the TFTs of the oxide semiconductors. In such a case theTFTs of the polysilicon semiconductors can be used in the peripheraldriving circuits.

On the other hand, the TFT of the polysilicon semiconductor has a higherleak current, therefore, a voltage change in the pixel electrode becomesa problem. Thus, it is reasonable to use the TFTs of the oxidesemiconductors in the pixels in the display area, and to use the TFTs ofthe polysilicon semiconductors in the peripheral driving circuit. Such astructure can be called as a hybrid type; the present invention can beapplied to the highbred type devices, too.

What is claimed is:
 1. A display device having plural pixels includingthin film transistors (TFT) having oxide semiconductor films comprising:a gate insulating film formed on the oxide semiconductor film, analuminum oxide film formed on the gate insulating film, a gate electrodeformed on the aluminum oxide film, a side spacer formed on both sides ofthe gate electrode, and an interlayer insulating film formed on the gateelectrode, the side spacer, a drain and a source, wherein in a planview, and in a direction from the drain to the source, a length of thegate electrode is shorter than a length of the aluminum oxide film. 2.The display device according to claim 1, wherein the oxide semiconductorfilm has a channel, which is formed between the drain and the source, anintermediate region is formed between the channel and the drain andbetween the channel and the source, a gate insulating film is formed onthe channel and the intermediate region of the oxide semiconductor film,and the gate electrode is formed on the aluminum oxide film above thechannel.
 3. The display device according to claim 2, wherein, in a planview, the channel is formed at a place where the gate electrode contactswith the aluminum oxide film and overlaps with the oxide semiconductorfilm, and the intermediate region is formed at a place where the sidespacer contacts with the aluminum oxide film and overlaps with the oxidesemiconductor film.
 4. The display device according to claim 2, whereinthe interlayer insulating film is directly in contact with the drain andthe source of the oxide semiconductor film.
 5. The display deviceaccording to claim 4, wherein a layer of the interlayer insulating filmthat is directly in contact with the drain and the source is made ofsilicon nitride.
 6. The display device according to claim 1, wherein acontent of hydrogen in the interlayer insulating film is higher than acontent of hydrogen in the side spacer.
 7. The display device accordingto claim 2, wherein a content of hydrogen in the oxide semiconductorfilm, is: in the channel<in the intermediate region<in the drain or thesource.
 8. The display device according to claim 2, wherein the channelhas a channel length and a channel width, provided a side of the sidespacer that is in contact with the aluminum oxide film is a bottom, anda side opposite to the bottom is a top, a length of the bottom in thechannel length direction is longer than a length of the top in thechannel length direction.
 9. The display device according to claim 8,wherein the length of the bottom of the side spacer in the channellength direction is by 0.3 microns or more longer than the length of thetop of the side spacer in the channel length direction.
 10. The displaydevice according to claim 1, wherein a height of the side spacer is 100to 500 nm.
 11. The display device according to claim 1, wherein a metalthat constitutes the gate electrode covers the side surface of the sidespacer and extends to a top of the side spacer.
 12. The display deviceaccording to claim 11, wherein a length, in the direction from the drainto the source, of the metal that constitutes the gate electrode on thetop of the side spacer is 0.1 microns or more.
 13. The display deviceaccording to claim 1, wherein a side surface, which contacts with theinterlayer insulating film, of the side spacer is a tapered surface. 14.The display device according to claim 1, wherein the drain and thesource of the oxide semiconductor film are covered by the gateinsulating film, and the gate insulating film contacts with theinterlayer insulating film.
 15. The display device according to claim14, wherein a layer of the interlayer insulating film that is directlyin contact with the gate insulating film is made of silicon nitride 16.The display device according to claim 14, wherein a content of hydrogenin the interlayer insulating film is higher than a content of hydrogenin the side spacer.
 17. The display device according to claim 14,wherein a content of hydrogen in the oxide semiconductor film, is: inthe channel<in the intermediate region<in the drain or the source. 18.The display device according to claim 14, wherein the channel has achannel length and a channel width, provided a side of the side spacerthat is in contact with the aluminum oxide film is a bottom, and a sideopposite to the bottom is a top, a length of the bottom in the channellength direction is longer than a length of the top in the channellength direction.
 19. The display device according to claim 1, whereinthe display device is an organic EL display device.
 20. The displaydevice according to claim 1, wherein the display device is a liquidcrystal display device.